In recent years, CO2 reduction is a big issue from a viewpoint of global environmental protection, and hence it is required to dramatically improve fuel efficiency in automobiles being a CO2 generating source. One of effective measures for the improvement of fuel efficiency is a weight reduction of an automobile body by using a high-strength steel sheet. However, the workability of the steel sheet is decreased as its strength is increased, so that it is required to improve the workability when the high-strength steel sheet is used for automotive members pressed into complicated shapes.
As one of addition elements effective for establishing both the increase of the strength and the improvement of the workability in the steel sheet is mentioned Si, which can be added in combination with C, Mn or the like to form a structure of steel having a high strength and an excellent workability, e.g., a structure of steel containing ferrite or bainitic ferrite and mixing with martensite or residual austenite.
Also, the pressed automotive members are commonly subjected to a phosphating treatment and then an electrodeposition coating after being assembled into an automobile body in order to improve the corrosion resistance. However, Si is known as a harmful element deteriorating the phosphatability of steel. Because the Si-containing steel sheet easily forms a Si-containing oxide on a surface of the steel sheet in its production process and such an oxide hinders phosphating reaction and causes such a problem that phosphating crystals to be formed uniformly and finely are coarsened or an area forming no phosphating crystal (what is called “lack of covering”) is generated.
As a measure for solving such a problem, for example, Patent Documents 1 and 2 propose techniques of manufacturing a high-strength cold rolled steel sheet with a phosphatability and hence a coating adhesion improved by controlling a ratio of Si content to Mn content (Si/Mn) to not more than 0.40 and further limiting a composition and an abundance of a metallic structure or a surface oxide.